1. FIELD OF THE INVENTION
The present invention relates to a moisture-proof electronic component mounting board and to a method of manufacturing the same.
2. DESCRIPTION OF THE PRIOR ART
The conventional electronic component mounting board has openings formed in the upper surface using an end mill thereof on which electronic parts are mounted and a heat radiating plate on the opposite surface thereof so that the heat from the electronic parts such as semiconductors may be radiated (for example, reference is made to Japanese Laid-Open Patent No. 32191-1984, and U.S. Pat Nos. 4,773,955 and 4,737,395).
Further, it has been usual that the above-mentioned heat radiating plate is bonded to a recess of the board with a bond. Although the electronic parts mounted on the board have their surfaces protected with a moisture-resistant sealing resin, yet moisture penetrates into the electronic parts through the bond. Therefore, it has also been practiced to cover with a metal plating layer the area exposed to the atmosphere, namely, the area between the side wall of the recess and the side wall of the heat radiating plate. The structure of such a board will be described with reference to FIGS. 19A through 21.
According to a conventional method of manufacturing an electronic part mounting board, a recess 92 (FIG. 19A) to which a heat radiating plate is bonded is first formed on a substrate 9 in the form of a copper-coated laminated plate as shown in FIGS. 19A through 19E. Next, the heat radiating plate 7 is, bonded to the surface 921 of the recess 92 with a bonding material 8 (FIG. 19B). Reference number 91 designates a metal layer formed of copper foil or the like.
Next, as shown in FIG. 19C, the upper surface of the substrate 9 is counter sunk until the heat radiating plate 7 is exposed to thereby form a mounting opening 93 for electronic parts. Then, as shown in FIG. 19D, a metal plating layer 76 is formed on the inner wall of the mounting opening 93 and on the exposed upper surface of the heat radiating plate, and another metal plating layer 75 is formed to cover the lower surface of the heat radiating plate 7 and the rear surface of the substrate 9.
After that, a pattern 94 is formed as shown in FIG. 19E to thereby provide an electronic component mounting board 90.
The board 90 thus obtained has the metal plating layer 75 covering the lower surface of the heat radiating plate 7 and the rear surface of the substrate 9 and the metal plating layer 76 covering the exposed part of the upper surface of the radiating plate 7 and the inner wall of the mounting opening 93 as shown in FIG. 19E. Therefore, penetration of moisture from the rear surface of the substrate 9 into the mounting opening 93 is hindered.
However, the above-mentioned electronic part mounting board 90 has sometimes had the defect that, as shown In FIG. 20, the portion (clearance) 95 between the side wall of the recess 92 and the opposing side wall of the heat radiating plate fails to be covered by the metal plating 75 with the formation of an opening 750 in that layer 75. That is, a hole 751 is formed due to an insufficient metal plating.
When such a defective hole 751 is formed, there is a danger that an electronic part such as a semiconductor is damaged due to penetration of moisture therefrom. Especially, when the substrate 0 is made of a resin type material, moisture penetrates into the recess 92 from the defective hole 751 and reaches the electronic part (not shown) via the resin material.
It is felt that the formation of the above-mentioned defective hole 751 depends on the size of the clearance 95 between the recess 92 and the heat radiating plate 7. That is, the larger the clearance 95 (e.g., about 0.1-0.2 mm), the larger the opening 750 resulting in discontinuity of the metal plating layer 75 thereat.
Therefore, it may be contemplated to reduce the size of the clearance 95. However, the clearance tends to be formed notwithstanding because of scatterings in the processing of recess 92 and the shaping of the heat radiating plate 7.
Further, as a countermeasure against the above-mentioned problems, there is considered a method in which, as shown in FIG. 21 recess 92 is made wider to enlarge the clearance between the side wall of the recess 92 and that of the heat radiating plate 7 at the reversed bottom surface 921 of the recess and then the metal plating layer 75 is continuously formed over the lower and side surfaces of the heat radiating plate, the side surfaces and reversed bottom surface 921 of the recess 92, and the rear surface of the substrate 9. Thus, according to this method, the metal plating layer 75 is formed to cover the above-mentioned clearance too, because the clearance 95 is sufficiently large.
However, the above method is disadvantageous in that the enlarged recess 92 hinders the freedom of formation of a pattern on the rear surface of the substrate 9. On the other hand, if the size of the recess 92 is limited, the heat radiating plate 7 will inevitably become small-sized resulting in hindering the heat radiating property thereof.
Moreover, in the above method, the bonding material 8 may sometimes overflow from between the bottom surface 921 of the recess 92 and the heat radiating plate 7 at the time of bonding. Thus, if the bond overflows too much, the metal plating layer 75 will not be sufficiently formed. Therefore, it becomes necessary to adjust the amount of bonding material 8 used.